PREFACE. vii 



or geological point of view, or as tending to increase our 

 imperfect knowledge of the habits and instincts of these 

 humble works of our Common Creator, very much yet 

 remains to be done. No lover of nature need share in 

 Alexander's sigh, or regret that he has no more work to 

 do in any department to which he chooses to devote 

 himself. " Nulla dies sine linea" was the favourite 

 maxim of the great Linne" ; and our days are too few for 

 the accomplishment of all that we propose to do, let our 

 aspirations be ever so modest. 



A few explanatory remarks as to the scope of this 

 work may be here introduced. 



It was at first my intention to give, in an Intro- 

 duction, a general outline of the subject, and to treat at 

 considerable length some of the principal topics which 

 are more especially interesting to naturalists and geolo- 

 gists. But the space which is necessarily occupied by 

 the body of the work (although the synonymy has been 

 compressed within the shortest limits and the descrip- 

 tive characters have been printed in small type) will not 

 admit of justice being done to this part of the subject, 

 without making the volume too bulky ; and I must there- 

 fore content myself with offering in the proper place a 

 few observations, so as to elicit further discussion. 



In describing the dimensions of shells, I have taken 

 the measurements from average and adult specimens, 

 dividing inches into decimal and centesimal parts ac- 

 cording to the size of each species. This mode of 



